I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a system and method for performing mobile switching center initiated service negotiation.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AM) schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated by reference herein.
In the wireless CDMA system, a large number of wireless subscriber units, each having a transceiver, communicates through satellite repeaters or terrestrial stations which are also referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographic area and routes calls carried over subscriber units to and from a telecommunication network via a mobile switching center. When a subscriber moves into the geographic area of a new cell, the routing of that subscriber's call may be eventually made through the new cell by a process called a "handoff."
A subscriber unit transmits a signal that is received by a base station. The signal is then relayed to a mobile switching center which in turn routes the signal to the public switched telephone network and to telephone lines or other subscriber units. Similarly, a signal may be transmitted from the public switched telephone network to a subscriber unit via a base station and a mobile switching center. The communications channel allocated for communication of information between the subscriber unit and the base station is called the traffic channel.
The interface between the subscriber unit and the base station is referred to as the air interface. The telecommunications industry association (TIA) has provided a standard for CDMA call processing on the air interface entitled "IS-95 Mobile Station--Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System." Addendum to IS-95 are provided as Telecommunications Service Bulletins (TSB). The standard IS-95+TSB74 includes provisions for service negotiation on the air interface and is incorporated herein by reference. The interface between the base station and the mobile switching center is referred to as the A interface. The TIA has provided for call processing on the A interface through the standard provided in "IS-634 Mobile Switching Center--Base Station Interface for Public 800 MHz," which is also incorporated herein by reference. IS-95-A and IS-634 both define the messages and signals that are sent on their respective interfaces for the operation of a CDMA wireless system.
The call flow in a CDMA system requires processing on both the air interface and the A interface. The successful progression of a call requires that the proper messages and signals are sent at the right times on both the air interface and the A interface. The IS-634 standard is being developed to provide for call processing on the A interface. A number of problems and deficiencies are present in IS-634 which currently does not support some of the necessary operations on the A interface.
One deficiency in IS-634 is in the handling of the delivery of Short Message Service (SMS) messages by the mobile switching center. The Short Message Service provides for the transfer of SMS messages between a subscriber station and a Message Center. The mobile switching center and the base station provide a conduit for the messages between the message center and the subscriber station. Generally, SMS messages are messages associated with services subscribed to by a user. For example, a user may subscribe to a "stock quotes" service. The user will then periodically receive the quotes in the form of SMS message. SMS messages may be displayed as text messages on the subscriber unit. The SMS messages are sent from the mobile switching center to a base station, which in turn transmits the SMS messages to a subscriber unit. The base station may transmit the SMS messages to the subscriber unit on the paging channel or the traffic channel.
A problem arises when a new call comes in for the subscriber unit while the subscriber unit is receiving SMS messages on the traffic channel. Since the traffic channel is already occupied for delivery of SMS messages, the only options available to the mobile switching center are to either provide a busy signal to the calling party, or drop the SMS call and reestablish the traffic channel to deliver the new call. Neither option is very desirable. This deficiency is recognized and solved by the present invention in the manner described below.